1. Field of the Invention
This invention pertains to the enhanced recovery of heavy crude oils and tars from subterranean formations containing same, and particularly to the injection of hot hydrocarbon vapors into the formation for heating the formation and extracting the oil therefrom using a single wellbore.
2. Description of Prior Art
Steam injection is the only enhanced recovery method for low gravity oils which is commercially practiced to a significant extent at the present time. The probable future of oil recovery by this method is described in the recent report "Enhanced Oil Recovery, An Analysis of the Potential for Enhanced Oil Recovery from Known Fields in the United States--1976 to 2000," National Petroleum Council, December 1976. Two basic techniques are generally used: steam stimulation involving alternate steam injection and oil recovery procedures (huff and puff) usually from the same hole, and steam drive of oil from injection wells to separate recovery wells. Both techniques are described in the above report on pages 13-15.
Unfortunately, steam injection for recovery of heavy oils is very expensive in energy due to the fact that it is necessary to heat a substantial portion of the subterranean oil bearing formation, and extraneous rock as well, to the saturation temperature of steam. The enhanced oil recovery effect is achieved substantially thermally by heating an entrapped oil to reduce its viscosity, and then physically by driving the oil out of the formation to a recovery well using differential steam pressure. The recovery of heavy oils using steam has been only partly successful, due in part to the immiscibility of water and oil. Some formations such as tar sands are totally impermeable to steam or other gases until the bitumen content has been extracted, whereupon they are very permeable. Other formations are subject to swelling and other damage.
A major problem with steam injection and with production of heavy oils generally is the tendency to form water-oil emulsions, which are difficult to break chemically and also have very high viscosities. Typically, for a heavy crude oil with a viscosity of 20,000 centipoise, the water-oil emulsion viscosity may reach 100,000 centipoise. Another major problem is establishing and maintaining satisfactory fluid communication between the injection and producing wells, particularly in formations such as tar sands which are usually essentially impermeable. Also in the "huff and puff" method of oil recovery using single wells, the alternate heating and cooling of the steel casing causes a major thermal expansion problem.
Light hydrocarbon diluents are widely used in the production of heavy oils to reduce viscosity and improve pumpability of the oil in the well, but these diluents are injected into the well as liquids to achieve this improvement. Injection of hydrocarbon diluents can also be used to repair formation damage caused by steam or water injection and can improve production by dissolving asphaltenes and other poorly soluble materials that are precipitated near the well bore. However, locating a supply of suitable diluent has become a major problem, and the diluent is a major cost in the production of heavy oil since it is sold as a mixture with the product, usually at a lower price than that of the diluent itself.
In the development of the present invention, it was unexpectedly found that these oil recovery problems can be overcome by the injection of a hot hydrocarbon solvent vapor into the formation rather than steam, using single wells. The general use of solvents in assisting production of heavy oils from formations is known. Also, solvents have been used in vapor form or partially vapor form and have achieved their effect by dilution and solubilization of the heavy oil. For example, U.S. Pat. No. 3,608,638 to Terwilliger discloses injecting various solvent vapors into oil formations such as tar sands to facilitate oil recovery from a separate production well spaced from the injection well. Other pertinent disclosures are provided by U.S. Pat. No. 3,515,213 to Prats and U.S. Pat. No. 3,695,354 to Dilgren, et al. for shale oil recovery from permeable zones in shale formations made permeable by fracturing and using various heated fluids.
Another major problem in production of heavy oils which frequently occurs in unconsolidated tar sands is "sanding" in the wells, which severely restricts the flow of oil from the formation into the well. Sanding problems are usually severely aggravated by steam and water injection, which mobilizes the adjacent sand and causes a considerable flow of sand and clay particles into the well along with the oil and water produced. I have shown in the development of the present invention that the use of solvent vapor inhibits such sanding and that a much cleaner oil is produced than with steam injection. Furthermore, the low viscosity of the oil-solvent mixture produced allows residual sediment to settle out efficiently, either in the well or in storage tanks above ground.
Accordingly, it is an object of this invention to provide a process for recovery of heavy crude oils and tars from subterranean formations by injecting a heated hydrocarbon vapor into the formation.
It is also an object of the invention to perform the vapor injection and oil recovery in single wells separated into injection and recovery portions or zones to avoid dependence on fluid communication between adjacent wells.
It is another object of the invention to reduce sanding of wells by using a compatible hydrocarbon vapor rather than steam or water injection.
It is still another object of the invention to recover the injected hydrocarbon vapor by a distillation step performed in the oil field, and to eliminate water from the oil and prepare it for shipment as a hot, dry liquid.
Other objects of the invention will become apparent in the description of the invention and preferred embodiments.